1. Field of the Invention
The present invention relates to a spark plug (an ignition plug) that ignites fuel by generating an electric spark in an internal combustion engine.
2. Description of the Related Art
A spark plug is known in which a metal shell is fixed by heat crimping at the outer periphery of an insulator that holds a center electrode (see, for example, JP-A-2003-257583). In the heat crimping, the metal shell in which the insulator is inserted is heated and in this state, the metal shell is plastically deformed by a compression load so that the metal shell is fixed to the insulator. Generally, the metal shell of the spark plug includes a polygonal-shape tool engaging section that engages with a tool to attach the spark plug to an engine head and a body section that compresses a gasket toward the engine head. A groove section that bulges to the outer peripheral direction and the inner peripheral direction by the heat crimping is formed between the tool engaging section and the body section of the metal shell that is heat crimped to the insulator.
[Patent Document]
    JP-A-2003-257583
3. Problem to be Solved by the Invention
In recent years, as one of various solutions to improve fuel consumption or decrease the exhaust gas of an internal combustion engine, a reduction in spark plug diameter has been investigated. However, a decrease in the strength of the metal shell is not sufficiently taken into consideration in relation to miniaturization of the spark plug. For example, at a portion in which the thickness in the radial direction from the groove section through the body section at the metal shell is thinned, the breaking strength is lowered according to the lowering of hardness through the influence of heat at the time of heat crimping. Thus, there is a problem in that when the metal shell is miniaturized at a reduction ratio in its present format, the breaking strength of the groove section at the metal shell is not capable of being sufficiently secured and a crack may generate at the groove section.